As quick drying printing ink, known are the ink compositions cured via radical polymerization due to irradiation with actinic rays. However, these ink compositions tend to suffer from curing inhibition in the presence of oxygen.
Recently, an ink composition cured via cationic polymerization with irradiation of actinic rays has been proposed. Proposed ink compositions contain an oxetane compound, an optical cationic polymerization initiator and a pigment, and may include an epoxy compound if needed. Since cationic polymerization is initiated by existence of cations generated from a cationic polymerization initiator by irradiation of, for example, UV rays, polymerization is not inhibited by oxygen. Accordingly, the polymerization is not necessary to be carried out under an inert atmosphere, and has the advantage that prompt and complete polymerization can be performed in air.
However, when a cationic polymerization initiator is added to a cationic polymerizable compound, for example, an oxetane compound or an epoxy compound, and the composition is stored over a long period, cations may spontaneously be generated from the cationic initiator even when no UV rays are intercepted, resulting in a viscosity increase and a gelling of the composition due to initiation of polymerization of the cationic polymerizable compound.
In order to resolve this problem, addition of various amine compounds to improve storage stability has been proposed (for example, see Patent Documents 1-5.).
However, prevention effect of polymerization of the cationic polymerizable compounds has not been fully acquired by the use of amines disclosed so far. In a case of an ink which contains a colorant, condense of the colorant tends to occur and the dispersion stability has not been fully enough. By addition of an amine, the energy required for curing the ink increases, and, when the amines disclosed so far are added to the ink composition, the curability of the ink tends to become relatively lower. Further, the ink composition added with the amines disclosed so far may have safety problems, namely, the ink composition relatively easily causes a rash, when in contact with bare skin.
(Patent document 1) Japanese Patent Publication Open to Public Inspection (hereafter referred to as JP-A) No. 5-132505
(Patent document 2) JP-A No. 6-184093
(Patent document 3) JP-A No. 2000-186079
(Patent document 4) JP-A No. 2001-200049
(Patent document 5) JP-A No. 2003-292606